Standardized Precipitation Index (SPI)
"The Standardized Precipitation Index (SPI) is a widely used index to characterize meteorological drought on a range of timescales. On short timescales, the SPI is closely related to soil moisture, while at longer timescales, the SPI can be related to groundwater and reservoir storage. The SPI can be compared across regions with markedly different climates. It quantifies observed precipitation as a standardized departure from a selected probability distribution function that models the raw precipitation data. [...]
SPI = (P - P*) / σp
where P = precipitation
p* = mean precipitation
σp = standard deviation of precipitation" (Keyantash, John & National Center for Atmospheric Research Staff, 2018)
"The Climate Data Guide: Standardized Precipitation Index (SPI)". Keyantash, John & National Center for Atmospheric Research Staff (Eds). Last modified 07 Aug 2018. https://climatedataguide.ucar.edu/climate-data/standardized-precipitati….
Accessed Mar 1, 2019.